ABOVE BOARD
Parallelism and Crosstalk I
Crosstalk between two parallel traces is caused by a combination of two effects:
Mutual Capacitive coupling - a current (IC) caused by capacitive coupling between two
traces and Mutual Inductive coupling - a current (IL) caused by inductive coupling between
two traces.
When considering the effects of crosstalk, it is important to consider these factors: The
degree of capacitive and inductive coupling between traces; the direction of the signals
and the reflection of the backward coupled signal.
Consider the traces illustrated here.
| IC + IL | IC - IL |
| A | ------------------------------------------------ | B |
| X | C | ------------------------------------------------ | D |
A signal propagates down the trace CD from point C to Point D. Assume the leading edge
of the rise time of the signal is at point x. A signal will be coupled into trace AB at x that
will travel in both directions. The forward coupled signal (Forward Crosstalk) is
proportional to the DIFFERENCE between the two coupling effects (IC-IL). The reverse
coupled signal (Backward Crosstalk), is proportional to the SUM of the two coupling
effects (IC + IL);
In normal PCB applications, the speed of the coupled pulses (i.e. the propagation time for
either the forward or backward crosstalk signal) will be the same as that of the driven
pulse on CD. For short duration pulses, the width of the forward crosstalk is equal to the
rise time of the driven pulse. The reverse pulse width however, (backward crosstalk) is
equal to twice the propagation time of the length of the coupled (parallel) line.
One way to visualize this is as follows :
Picture a train moving from C to D whose engine is just passing C and pushes a "bow
wave" (analogous to the rise time of the pulse). Forward crosstalk starts at A and moves
towards B at the same speed as the engine. Backward crosstalk has also been generated
which reflects off of A and is also moving towards B. As the engine continues towards D, it
continues to couple a signal in the parallel line that travels backwards towards A and then
reflects back to B again. This continues until the engine reaches D.
The forward crosstalk continues to build over the entire length, so the magnitude of the
forward crosstalk signal is proportional to the coupled length. However, since the inductive
and capacitive effects tend to cancel in normal PCB applications (since the materials are
relatively homogeneous) the signal still tends to be very small.
The backward crosstalk signal, however, reaches a maximum value at the point where the
coupled length is tr/2tpd or greater. The magnitude of this signal is hard to predict. One
study suggests that a maximum magnitude of about 20% for 2nsec rise time signals in close
coupled lines in Microstrip and 12% for 2nsec rise time signals in close coupled lines in
Stripline are reasonable estimates. Increased separation and signal reflections will reduce
the size of the signal.
Finally, the device impedance at point A is usually relatively lower than the intrinsic impedance of the trace. So the backward crosstalk (voltage) signal will reflect at point A back towards with a negative reflection coefficient and a magnitude that will depend on the reflection coefficient
| p = | (RL-ZO) |
| (RL+ZO) |
The reflection coefficient would normally be relatively high (say .5 to .8).
Continued in Part II
UltraCad Design in Bellevue Wash., specialize in designing high speed boards and can be reached at (206)450 9708.